2 Answers from Attorneys

No, but if the 18 year old is pulled over the car would smell like alcohol and the officer would certainly suspect the 18 year old of drinking.... Expect some field sobriety tests if you get pulled over. And if you're not drinking and you are asked to blow in the breathalyzer, do not refuse. Remember, any alcohol in your system (if you are under 21) would be an arrestable offense

What Mr. Jacobs said is entirely true. If you were not drinking (and had nothing else in your system either) then you would want to insist upon a test of your blood, breath or urine to prove your innocence.

But the best practice here is not to put yourself into this situation. It may cost a great deal of money to resolve the case in your favor.